#### Abstract

A novel experimental approach based on a toroid Cavendish balance is used to evaluate the product of photon mass squared and the ambient cosmic magnetic vector potential $A$. The method is based on the energy density of the vector potential in the presence of photon mass, not on measurement of the magnetic field. The experiment discloses $A{\mu}_{\gamma}^{2}<2\mathrm{}\times {10}^{-9\mathrm{}}\phantom{\rule{0ex}{0ex}}\mathrm{T}\mathrm{m}/{\mathrm{m}}^{2}$, with ${\mu}_{\gamma}^{-1\mathrm{}}$ as the characteristic length associated with photon mass. Consequently, if the ambient magnetic vector potential is $A\approx {10}^{12}\phantom{\rule{0ex}{0ex}}\mathrm{T}\mathrm{m}$ due to cluster level fields, ${\mu}_{\gamma}^{-1\mathrm{}}>2\mathrm{}\times {10}^{10}\phantom{\rule{0ex}{0ex}}\mathrm{m}$. If we conservatively use galactic fields prior to a reversal, then ${\mu}_{\gamma}^{-1\mathrm{}}>1\mathrm{}\times {10}^{9}\phantom{\rule{0ex}{0ex}}\mathrm{m}$, a figure still superior to that derived from the Jovian magnetic field.

DOI: http://dx.doi.org/10.1103/PhysRevLett.80.1826

- Received 2 June 1997
- Revised 11 December 1997
- Published in the issue dated 2 March 1998

© 1998 The American Physical Society